docs: def assume_role(credentials, account, role):
      """Use FAWS provided credentials to assume defined role."""
      sts = boto3.client(
          'sts',
          aws_access_key_id=credentials['accessKeyId'],
          aws_secret_access_key=credentials['secretAccessKey'],
          aws_session_token=credentials['sessionToken'],
      )
      resp = sts.assume_role(
          RoleArn='arn:aws:sts::{}:role/{}'.format(account, role),
          RoleSessionName='fleece_assumed_role'
      )
      return {
          'accessKeyId': resp['Credentials']['AccessKeyId'],
          'secretAccessKey': resp['Credentials']['SecretAccessKey'],
          'sessionToken': resp['Credentials']['SessionToken'],
      }